Electrochemical energy storage is currently used in many different portable applications, such as wireless communications and portable computing, just to name a few, and will be essential for the realization of future fleets of electric and hybrid electric vehicles, which are now believed to be an essential part of the world's strategy for reducing our dependence on oil and minimizing the impact of gaseous emissions of CO and CO2 on climate change. In looking at possible materials that can be used for anodes in electrochemical energy conversion and storage systems, lithium appears to have one of the highest specific capacities, in terms of Ah/kg. See FIG. 1 which is a plot showing rankings of conventional anode materials. Hydrogen is typically used to power fuel cells, while lithium is typically used in advanced rechargeable battery cells and batteries.
Most currently used energy storage systems use lithium ion battery chemistry, with graphite anodes that intercalate lithium upon charging, mixed transition metal oxide cathodes that intercalate lithium during discharge, a micro-porous polyethylene electrode separator, and an electrolyte formed from a dielectric mixed solvent composed of organic carbonates, other solvents and high-mobility lithium salts. The movement of the lithium ions between the intercalation anodes and cathodes during charging and discharging is commonly known as the “rocking chair” mechanism.
Cells with liquid electrolytes are usually housed in cylindrical or prismatic metal cans, with stack pressure maintained by the walls of the can, while cells with polymer gel electrolytes are usually housed in soft-sided aluminum-laminate packages, with stack pressure achieved through thermal lamination of the electrodes and separators, thereby forming a monolithic structure.
Graphite powder is used as the active material for anodes, is coated onto thing copper foils that serve as current collectors for the anodes, and is held in place by a polyvinylidene fluoride (PVDF) binder. Transition metal oxide powder is used as the active material for cathodes, and is coated onto thin aluminum foils that serve as current collectors for the cathode, and is held in place by a PVDF binder. Both natural and manmade graphite, such as mesocarbon microbeads (MCMB), have been used for the anodes, while LixCoO2, LixNiO2, LixMn2O4, mixed transition metal oxides with cobalt, nickel, and manganese, and iron-phosphates, among others, are common choices for the cathode.
Over the past decade, these systems have attained outstanding specific energy and energy density, exceptional cycle life and rate capabilities that enable them to now be considered for both vehicular and power tool applications, in addition to their early applications in wireless communications and portable computing. The best commercially available, polymer-gel lithium ion battery now has a specific energy of greater than 180 Wh/kg, an energy density of greater than 360 Wh/L, and a reasonably good rate capability, allowing discharge over a broad range of C-rates.
Both liquid prismatic and polymer gel cells may be incorporated into large high-capacity power packs for electric vehicle and other applications. Such high capacity systems have state-of-the-art computerized charge and discharge control systems which include graphical user interfaces, and are capable of sensing for monitoring the health of individual cells, and balancing the charge of individual cells in large series-parallel arrays of cells.
Such lithium ion batteries, which rely on the rocking chair mechanism, are generally believed to be safer than those where lithium exists in the reduced metallic state. However, the use of flammable liquid-phase and two-phase polymer gel electrolytes, coupled with a high energy density, a relatively delicate about 20 micron thick polymeric separator, and the possibility of lithium plating and dendrite formation due to non-uniform stack pressure and electrode misalignment, has led to safety problems with these energy storage systems. One example of the type of unanticipated event with a lithium ion battery is evidenced by the rash of laptop battery fires experienced over recent years. The possibility of such an event occurring on commercial airliners, where many passengers carry laptop computers and cell phones with such batteries, is especially disconcerting. These events have occurred on much larger scales, and have caused industry-wide concern in the continued use of this important technology.
Adequate and intelligent thermal management in these cells is essential. High rates of charge or discharge drive the temperature upward due to resistive heating of the electrolyte. When the core temperature of these cells exceed a critical threshold (typically about 150° F.), the systems frequently become unstable, with the possible initiation of autocatalytic reactions, which can lead to thermal runaway and catastrophic results. Disproportionation of the transition metal oxides can liberate sufficient oxygen to support oxidation of the organic carbonate solvents used in the liquid or polymer-gel electrolytes. It is now recognized that while conventional systems provide high energy density, their safety remains problematic.
It would therefore be very beneficial to develop new battery materials and architectures that enhance the performance of rechargeable solid-state lithium-ion batteries, and that will provide high specific energy, high volumetric energy density, and high rate capability at high and/or low temperatures, e.g., about 0° C., with substantially improved safety and reliability through the elimination of combustible liquid organic solvents, to the greatest extent possible.
The battery industry has become extremely competitive, with lower prices placing pressure on battery manufacturers to optimize production processes, eliminating as many unnecessary production steps as possible. Great economic advantage could be achieved through reducing the number of steps involved in coating electrodes and fabricating separators, for example. Furthermore, it would be beneficial to construct cells in a bipolar architecture, further eliminating the weight and cost associated with electrode interconnects, with the possibility of a higher cell voltage than otherwise possible.